(a) Field of the Invention
The present invention relates to a method for depositing selenium alone or a mixture of selenium and a minor amount of additives on an appropriate substrate in vacuum, and also to selenium sources for use in such vacuum-deposition. More particularly, it relates to a method for performing vacuum-deposition, into a film form, of selenium or doped selenium on a support for use in the production of a photoconductive-layer-containing electrostatographic plate, a selenium rectifier and a selenium photocell using a specific selenium source.
(b) Description of the prior art
Generally, selenium metal is produced as a byproduct of copper refining process. More particularly, during the process of refining copper electrolytically, precipitate is formed in an electrolytic cell. This precipitate is roasted and thus selenium dioxide is producted. This selenium dioxide is then dissolved in water to form an aqueous solution thereof. By introducing a reducing agent such as gaseous sulfur dioxide into this solution, fine particles of selenium precipitate on the bottom. This precipitate is removed from the solution and is washed with water, and then dried with heated air. In this way, the powdery metallic selenium is obtained. Though the selenium thus produced has a considerably high purity, its powdery state is inconvenient in handling and apts to cause a loss of selenium due to the tendency to scatter in the ambient air, causing the problem of environmental pollution. To avoid such inconveniences, it has been the practice to melt the powdery selenium and then quickly cool the melt. The resulting amorphous selenium granules have been commercially marketed.
The granules of amorphous selenium have been widely used as a source selenium material in performing vacuum-deposition of selenium. Typically, the particulate amorphous selenium may be produced by melting powdery metallic selenium in a crucible at a temperature of about 300.degree. C., dropping the melt into water through a screen with meshes of about 1 mm to quench and solidify the particles of selenium which have become amorphous and now have a particle size of about 1 mm. Such amorphous selenium, however, has a relatively low purity in most cases, because various contaminants are apt to be introduced in the selenium during the steps of melting and quenching in water which are employed in the production process thereof. Moreover, the particulate amorphous selenium begins to soften when heated at a temperature of about 40.degree. C. or higher, and agglomerates making it difficult to degas the particles. The entrapped gases are released during the subsequent vacuum deposition process, and as a result, the film formed by the deposition tends to have a poor quality. Moreover, in some cases, the gaseous contaminants adsorbed on the selenium particles in the course of the granulating process may burst out from a source containing the selenium during vacuum deposition, resulting in fracture of the surface of a film of selenium deposited. In addition, during the deposition process, the vacuum chamber may be contaminated by the impurities adsorbed on the amorphous selenium. Furthermore, metallic selenium may, to some extent, be evaporated during the melting step of the granulating process, and the dispersed selenium vapor may cause environmental pollution. In this way, the prior art using the particulate amorphous selenium has many inconveniences and disadvantages as to the quality of the film formed by deposition and to environmental sanitation.